The lovers' holiday traces its roots to raucous annual Roman festivals where men stripped naked, grabbed goat- or dog-skin whips, and spanked young maidens in hopes of increasing their fertility, said classics professor Noel Lenski of the University of Colorado at Boulder.

The annual pagan celebration, called Lupercalia, was held every year on February 15 and remained wildly popular well into the fifth century A.D.—at least 150 years after Constantine made Christianity the official religion of the Roman Empire.

"It is clearly a very popular thing, even in an environment where the Christians are trying to close it down," Lenski said. "So there's reason to think that the Christians might instead have said, OK, we'll just call this a Christian festival."

The church pegged the festival to the legend of St. Valentine.

According to the story, in the third century A.D. Roman Emperor Claudius II, seeking to bolster his army, forbade young men to marry. Valentine, it is said, flouted the ban, performing marriages in secret.

For his defiance, Valentine was executed in A.D. 270—on February 14, the story goes.

While it's not known whether the legend is true, Lenski said, "it may be a convenient explanation for a Christian version of what happened at Lupercalia."

Valentine's Day 2009: What Recession?

Even as the economy crumbles, today's relatively tame Valentine's Day celebration is expected to generate some $14.7 billion in retail sales in the United States.

The average U.S. consumer is expected to spend $102.50 on Valentine's Day gifts, meals, and entertainment, according to an annual U.S. National Retail Federation survey—down from $122.98 per person in 2008.

"If anything, [people] are probably scaling back on more discretionary purchases, so that they can feel comfortable spending on Valentine's Day," said Ellen Davis, the federation's vice president.

About 92 percent of married Americans with children will spend the most money on their spouses: $67.22.

The remainder goes to Valentine's Day gifts for kids, friends, co-workers, and pets, according to the survey.

Valentine's Day Cards

Greeting cards, as usual, will be the most common Valentine's Day purchases. Fifty-eight percent of American consumers plan to send at least one, according to the survey.

The Greeting Card Association, an industry trade group, says 190 million Valentine's Day cards will be sent. And that figure does not include the hundreds of millions of cards schoolchildren exchange.

"Giving your sweetheart or someone [else] a Valentine's Day card is a deep-seated cultural tradition in the United States," said association spokesperson Barbara Miller. "We don't see that changing."

The first Valentine's Day card was sent in 1415 from France's Duke of Orléans to his wife when he was a prisoner in the Tower of London following the Battle of Agincourt, according to the association.

Valentine's Day cards—mostly handwritten notes—gained popularity in the U.S. during the Revolutionary War. Mass production started in the early 1900s.

Hallmark got in the game in 1913, according to spokesperson Sarah Kolell. Since then—perhaps not coincidentally—the market for Valentine's Day cards has blossomed beyond lovers to include parents, children, siblings, and friends.

Valentine's Day Candy: Cash Cow

An estimated 45.8 percent of U.S. consumers will exchange Valentine's Day candy, according to the retail federation survey—adding up to a sweet billion dollars in sales, the National Confectioners Association says.

About 75 percent of that billion is from sales of chocolate, which has been associated with romance at least since Mexico's Aztec Empire, according to Susan Fussell, a spokesperson with the association.

Fifteenth-century Aztec emperor Moctezuma I believed "eating chocolate on a regular basis made him more virile and better able to serve his harem," she said.

These are brain systems, not phases, Fisher emphasized, and all three play a role in love. They can operate independently, but people crave all three for an ideal relationship.

"I think the sex drive evolved to get you out there looking for a range of partners," she said.

"I think romantic love evolved to enable you to focus your mating energy on just one at a time, and attachment evolved to tolerate that person at least long enough to raise a child together as a team."

Valentine's Day, Fisher added, used to encompass only two of these three brain systems: sex drive and romantic love.

But "once you start giving the dog a valentine, you are talking about a real expression of attachment as well as romantic love."